Copyright © 1985 R.D. Eager
Permission is granted to copy and/or modify this document for private use only. Machine readable versions must not be placed on public web sites or FTP sites, or otherwise made generally accessible in an electronic form. Instead, please provide a link to the original document on the official ML/I web site.
This implementation of ML/I contains all the features described in the ML/I User's Manual, 4th Edition, August 1970, plus New Features 1 to 4 as described in supplements to that Manual.
The program has been implemented on GEORGE 3, but in such a way that it will also run under GEORGE 4 without change.
ML/I is usually run by calling the ML1 job control macro.
This macro takes several different types of parameter;
each type is identified by a different keyword.
The possible keywords are:
IN
IN). The first such file is designated the
primary input stream; the second is designated the
secondary input stream, and the third is designated the
tertiary input stream.
If the IN keyword is given without a value, or omitted,
the appropriate ML/I file is ONLINEd.
OUT
OUT). The first such file is designated the
primary output stream,
and the second is designated the secondary output stream.
If the OUT keyword is given without a value, or omitted,
then the appropriate ML/I file is ONLINEd.
DEBUG
DEBUG keyword is given without a value, or omitted, then the
debugging file is ONLINEd.
See Chapter 6 of the ML/I User's Manual
for a description of the uses of this file.
CORE
ENV
S18 is set
nonzero at the start of the run; this enables the context print-out
at the end of processing. See Section Q.7.2 for further details.
From the above, it can be seen that ML/I has three possible input streams and two possible output streams. There is also a stream for debugging messages.
ML1
This is sufficient for working through the Simple Introductory Guide.
PIG and DOG,
and output to FOO:
ML1 INPIG,INDOG,OUTFOO
Input may be taken from any one of the input streams. The value of
S10 controls the selection. The possible values are
S10 = 1
S10 = 2
S10 = 3
If S10 is set to zero, ML/I treats this as "end of file" and ceases
processing. If S10 is set to any illegal value (negative,
or greater than three) then the process is aborted.
If a change of input stream is made, the original stream is not
"forgotten". Any attempt to read from this stream again will cause ML/I
to carry on where it left off. When the end of an input stream is
reached, ML/I checks to see if it is the revert stream.
If it is, the process is terminated; otherwise input is switched to the
revert stream, and processing continues. The revert stream is initially
1; its value is held in S23 and may be altered by the user if required.
No line of input may exceed 160 characters.
Normally, no special end of file marker is necessary. However, it may
sometimes be useful to be able to force end of file from (for instance)
a MOP terminal. A record consisting of four asterisks will have the
same effect as setting S10 to zero, but will be recognised even when
ML/I is searching for a secondary delimiter
(in this situation, a MCSET to change the value of S10
would not be actioned until the construction was completed).
Sometimes, this facility may be a nuisance. To stop any terminator from
being recognised, S25 should be set to zero (its initial value is one).
To select an alternative terminator, S26 should be set to the decimal
value of a word containing the four characters required as a terminator.
It is possible to designate that one character be translated to another on input. This makes it possible to input a character that a device does not support (e.g. an underline or backarrow from a MOP terminal). However, only one character code can be translated in this way.
If it is desired to perform a translation, S16 should be set to the
ASCII code of the character to be translated,
and S17 to the ASCII code of the character that is to
replace it. For example, if @ (code 32) was to represent underline
or backarrow (code 63), S16 and S17 should be set in the
following way:
MCSET S16 = 32 MCSET S17 = 63
Initially, S16 has the value -1,
which since it does not correspond
to a valid internal code, will not cause any translations to be made.
The ordering of input operations is as follows:
S10 equal to zero.
S10.
S16 and S17.
Output may be directed to either, both or neither of
the primary and secondary output streams.
The values of S21 and S22 control the selection;
S21 controls output to the primary output stream,
and S22 controls output to the secondary output stream.
In both cases, a value of 0 means that no output is to take place,
and a value of 1 means that output is to take place.
No output line may exceed 160 characters in length.
ML/I uses all additional core allocated to it for workspace.
The CORE parameter to the ML1 macro is usually used to set this;
there is no practical limit to the amount of core that ML/I can use.
ML/I uses the standard 1900 GRAPHIC character set. Since all 64 character codes represent valid characters, the error character is not used.
Error messages are output to the debugging file specified in the call of ML/I; this defaults to the MOP terminal or job journal. With reference to Chapter 6 of the ML/I User's Manual, the number 2N (the maximum number of characters inserted into an error message without truncation) is 64.
A count of processing errors (i.e. occurrences of the word
ERROR(S) on the debugging file) is maintained in S11.
At the end of a process, ML/I checks this value;
if it is zero, ML/I terminates with the message:
DELETED: OK
If S11 is nonzero, the message given on deletion is:
DELETED: ER
In both cases, a suitable message is output by the ML1 macro.
The user is free to adjust the value of S11 whilst ML/I is processing,
in order to obtain special effects (e.g. forcing the ER deletion).
An output lines limit is imposed on the debugging file, to curb
excessive output from a process that has gone badly wrong. The limit is
implemented by holding a quota of "lines left" in S12;
if S12 ever goes negative, the process is aborted.
S12 is initially 500, but may be changed by the user.
At the end of a process, a message of the form
AT END OF PROCESS: N LINES, M CALLS
is output to the debugging file, followed (if S18 equals 1)
by a list of the currently defined constructions.
The following run-time messages are peculiar to this implementation. They may be followed by other, advisory, messages which are self-explanatory.
Message
DEBUGGING FILE LINES QUOTA EXHAUSTED
Description
The value of S12 (the quota of remaining lines allowed to the
debugging file) has become negative.
System Action
The current process is aborted.
Message
S10 HAS ILLEGAL VALUE, VIZ n
Description
S10has been set to the value n, which is outside the range 1-3. Note that this error may be caused byS23(the revert stream) being set to an illegal value, and end of file then being reached on another input stream.
System Action
The current process is aborted.
Message
PRIMARY OUTPUT FAILURE
Description
An error has occurred while writing to the primary output file
(*FH1).
System Action
The current process is aborted.
Message
SECONDARY OUTPUT FAILURE
Description
An error has occurred while writing to the secondary output file
(*FH2).
System Action
The current process is aborted.
The initial environment contains ten permanent variables, all set to zero. All integers in, or derived from, macro expressions should be less than 8388607 in magnitude. Overflow is not detected, except in the case of division by zero, and its effect is undefined.
The following are the layout keywords for this implementation:
SPACE | meaning a space.
|
NL | meaning a newline.
|
TAB | meaning a tab.
|
SL | meaning the imaginary startline character.
|
SPACES | meaning a sequence of one or more spaces.
|
There are 26 S-variables. S1 to S9 are independent of the
implementation, and are used to control and monitor ML/I itself.
S10 to S26 are implementation dependent, and are used to
control input/output, etc. If an S-variable is set to any value other
than those given below, the effect is undefined (except for invalid
values of S10, which always cause the process to be terminated).
S1-S9S1
S1 is one, the imaginary startline
character is inserted on input. If S1 is zero, no startlines are
inserted; this is the initial setting.
S2
S2; it may be changed at any time.
S3
S3 is one, the error message normally
generated if a warning marker is not followed by a macro name is
suppressed. If S3 is zero (the initial value), the message is
produced.
S4
S4 is one, the context print-out
normally given after a call of MCNOTE is suppressed. If
S4 is zero, the context print-out is given; this is the initial
setting.
S5
S6
S7
S8
S9
S10-S26S10
S10 is 1.
S11
S12
S12 contains the quota of lines on the debugging file. It is
initially 500, and every time ML/I outputs a line to the debugging file
(whether via an error message or a MCNOTE) it decreases
S12 by one. If S12 ever becomes negative, the process is
aborted. The user is at liberty to adjust the value of S12 at any
time.
S13
S14
S15
S16
S16 are translated to characters with the code given by
S17, on input. Initially S16 is -1, so no
translations are performed.
S17
S16 above.
S18
S18 is one at the end of a process, a list is given of all
currently defined constructions. This is output to the debugging file
(and is not subject to the quota of lines imposed by S12). If
S18 is zero, the list is not produced. The initial value of
S18 is determined by the setting
of bit 18 in the switch word, but is normally zero.
S19
S19. It may
be changed if desired.
S20
S21
S21 controls output to the primary output stream;
see Section Q.2.4 for details. Its initial value is 1.
S22
S22 controls output to the secondary output stream;
see Section Q.2.4 for details. Its initial value is 0.
S23
S23 contains the current revert stream. See Section Q.2.3 for
details.
S24
S24 contains the number of words of workspace available to ML/I at the
start of processing. It is not subsequently updated,
and is provided only so that macro packages may make decisions based
on the availability of workspace.
S25
S25 has the value one, then the four characters represented
by the contents of S26 are treated as an input terminator.
If S25 is zero, this facility is disabled.
The initial value of S25 is 1.
S26
S26 contains a number which represents the character codes of exactly
four characters which are to be treated as an input
terminator if they appear as the contents of an input record. Their effect
is controlled by the value of S25.
The initial value of S26 represents four asterisks (****).
Although ML/I is normally run by using the ML1 macro,
users may wish to write their own job control macros.
This Section gives a full specification of the interface to the ML/I
program itself.
The program is conventionally called KML1.
There is only one valid entry point, entry point 0 (location 20).
All input/output is performed using GEORGE "file handlers". The allocation of units is as follows:
*FH0
*FH1
*FH2
*FH3
*FH4
*FH5
All input/output record buffers are 160 characters in length, except for the debugging file (the limit here being 72 characters).
The following program switches control initial settings of certain S-variables:
Switch 18
ON, then S18 is initialised to 1;
the default setting is that the switch is OFF,
and in this case S18 is initialised to zero.
The following display is output at the start of each run of ML/I. It gives
the version number of the 1900 implementation of the program; the version
number of the main ML/I logic is given as part of the environment printout
controlled by S18 (see Section Q.7.2).
DISPLAY: ML/I (VERSION x.x) PROCESSING
ML/I may halt for several reasons. The meaning of the various halts (usually trapped and interpreted by job control macros) is as follows:
HALTED: ST
HALTED: ER
HALTED: IE
HALTED: FnRESUMEd.
HALTED: TnRELEASEd;
the user may assign another file and RESUME the run.
HALTED: En
ML/I deletes itself at the end of a run. One of two messages may be given:
DELETED: OK
S11 was zero at
the end of the run.
DELETED: ER
S11 was nonzero
at the end of the run.
See Section Q.4 for details of the use of S11 in detecting errors.